Ferrite Toroid Core: Nickel-Zinc Ferrite Sintered at 1300 °C in Oxygen (Snoek)

Ferrite is a ceramic magnet: iron oxide combined with other metal oxides, which carries magnetic flux but hardly conducts electricity. Snoek at Philips (US 2,452,530, 1948) made soft ferrite ring cores for coils and transformers. This is his nickel-zinc ferrite: nickel, zinc and iron oxides sintered twice at 950 °C, ground, pressed into a ring and fired at 1300 °C in oxygen. His ring reached an initial permeability of 3000. Nickel oxide dust is a cancer hazard: respirator on when milling. The picture: the patent's drawing (Fig. 4, the ring core).